1. Field of the Invention
This invention relates to a method of recovering metals from waste liquors and more particularly to a method of recovering silver from solutions containing small amounts of silver or silver containing components.
2. Description of Related Art
British patent specification No 499201 teaches a method of recovering metals from waste liquids such as those containing ores or those waste liquids obtained in the photographic industry. According to the process of this patent specification, an alkaline solution containing the small portion of silver containing material is treated with aluminum sulfate in order to form an aluminum hydroxide floc that carries down the sludge as a precipitate and then the volume of the sludge is reduced by the addition of an acid. In working up the thus treated sludge, aluminum sulfate is recovered when sulfuric acid is used in the acid treatment. This method is one commonly employed for recovering silver from waste solutions used in the photographic process and also for recovering silver from scrap film and paper which has previously been coated with silver salt containing emulsions.
A problem however has been encountered mainly in the recovery of silver from scrap film and scrap paper. Upon reducing the volume of the waste liquor by the addition of sulfuric acid the formation of a highly elastic residue is observed. Two problems are encountered in the further processing of these elastic residues. The first is that because of their elasticity they are difficult, if not impossible, to convey. Secondly, if they are employed in the next phase of the silver recovery process, which is customarily a roasting operation in order to drive off water and organic materials, they cause explosions.
While applicants do not wish to be bound by any theory with respect to the formation of these elastic residues, it is believed that they are due to the presence in the scrap film and paper of various complicated organic structures including the presence of photographic couplers and coupler solvents therefor. However, the formation of these elastic residues in the recovery of silver from photographic waste materials renders it necessary to provide a means for recovery of the silver contained therein which is not subject to either the formation of the elastic residues or the risk of explosion in the step of the process wherein water and organics are separated from the residue in roasters.